The present invention relates to an image processing apparatus, image processing method, image display apparatus, and image display method, and in particular to a technique for tone resolution expansion (i.e., tone resolution refinement) of digital images. The term “tone resolution expansion” is used to mean increase in the number of gray scale levels or tone levels, i.e., conversion of image data for each pixel represented by a first number of bits, to image data for each pixel represented by a second number of bits, the second number being greater than the first number. The “tone level” or “gray scale” refers to both color data and black/white data.
Various forms of tone level conversion or tone conversion are applied to the image, as part of the image processing. A problem associated with the tone conversion is occurrence of “tone jump.” The “tone jump” (which may also be called a tone value skip) is a phenomenon in which the image level varies step-wise between adjacent regions (or between adjacent pixels), and least one image level (tone value) is skipped or is absent. See for instance Japanese Patent Application Kokai Publication No. H10-84481 (Page 3, FIGS. 1 and 2.)
When an analog image signal is converted into a digital image signal, continuous tones become discrete by the quantization. When the resolution of the quantization is low (the number of the bits is small), there will be a greater number of tones which do not appear after the quantization, and the degradation of the image quality due to digitization is noticeable. If the resolution of the quantization is made high, the degradation of the image quality is alleviated, but the A/D converter used for the digitization becomes expensive.
According to the above-mentioned publication, a tone jump is perceived to the human eye as a pseudo contour.
A conventional analog-to-digital conversion is associated with a problem that, when an analog image signal having a gray scale or tone varying gradually is converted to a digital image data, the tone in the digital image data varies stepwise by “1, ” so that the degradation in the image quality due to the quantization is prominent. For instance, when image data of an evening sunset glow, sea, or the like is converted to the digital image data, the part where the tone of the digital image data varies stepwise by “1” appears to be a pseudo contour.
Also, when non-linear tone conversion, e.g., gamma conversion for compensating the non-linearity of the input signal-to-light emitting intensity conversion characteristics of a display device, is applied to the digital image data, there may be tones (tone values) which do not appear in the image data after the non-linear tone conversion.